As drilling technology improves, longer and longer wells are being drilled. The worlds' longest drilled oil well is BD-04-A, completed in May 2008 by Maersk Oil Qatar and Qatar Petroleum, in the Al-Shaheen offshore oil field off the coast of Qatar. The well includes a horizontal section measuring 35,770 ft (more than 6 miles).
Increasing well-reservoir contact via longer wells has a number of potential advantages in terms of well productivity, drainage area, sweep efficiency and delayed water or gas break-through. However, long wells not only bring advantages, but also present new challenges in terms of drilling, completion and production.
One of these challenges is the frictional pressure losses increase with well length. The inflow profile becomes distorted so that the heel part of the well produces more fluid than the toe when these losses become comparable to drawdown. This inflow imbalance, in turn, often causes premature water or gas breakthrough, which is highly undesirable.
Installation of an Inflow Control Devices or ICDs is an advanced well completion option that provides a practical solution to this challenge. An ICD is a device that directs the fluid flow from the annulus into the base pipe via a flow restriction. This restriction can be in form of tubes, helical channels, nozzles/orifices or a hybrid design (FIG. 1A-D).
In all cases, the ability of an ICD to equalize the inflow along the well length is due to the divergence in the physical laws governing fluid inflow in the reservoir and through the ICD. Liquid inflow in porous media is normally laminar, hence there is a linear relationship between the flow velocity and the pressure drop. By contrast, the flow regime through an ICD is turbulent, resulting in a quadratic velocity/pressure drop relationship.
The physical laws of flow through an ICD make it especially effective in reducing the free gas production. In situ gas viscosity under typical reservoir conditions is normally at least an order of magnitude lower than that of oil or water; while in situ gas density is only several times smaller than that of oil or water. Gas inflow into a well will thus dominate after the initial gas breakthrough if it is not restricted by gravity or an advanced completion.
ICDs introduce an extra pressure drop that is proportional to the square of the volumetric flow rate. The dependence of this pressure drop on fluid viscosity is weak for channel devices and totally absent if nozzle or orifice ICDs are used. These characteristics enable ICDs to effectively reduce high velocity gas inflow. The magnitude of a particular ICD's resistance to inflow depends on the dimensions of the installed nozzles or channels. This resistance is often referred to as the ICD's “strength.” It is set at the time of installation and cannot be changed without a major intervention to recomplete the well.
ICDs have been installed in hundreds of wells during the last decade, being now considered to be a mature, well completion technology. Steady-state performance of ICDs can be analyzed in detail with well modeling software.
We have found that ICDs work well in SAGD wells because of the phenomenon of steam blocking. The velocity of fluids increases when steam begins to break through and the differential pressure across the ICD (ΔP) increases, effectively blocking the steam from being produced. The problem is that all available ICDs were designed with the conventional oil production in mind, not SAGD or the many variants thereon, such as fishbone SAGD, radial SAGD, Cross SAGD (XSAGD), single-well SAGD (SW-SAGD), expanding solvent SAGD (ES-SAGD), Steam And Gas Push (SAGP), SAGD wind down, Fast SAGD, as well as in other enhanced recovery methods, such as Cyclic Steam Stimulation (CSS), High pressure cyclic steam stimulation (HPCSS), Vapor Extraction (Vapex), and the like.
Thus, what is needed in the art are methods of optimizing ICD design for these various enhanced oil recovery techniques.